A display device in which pixels are included by light-emitting elements formed of a light-emitting layer including organic electroluminescence materials has been developed as a flat plate display device. This display device is provided with a circuit (hereinafter referred to as “pixel circuit”) for controlling the light emission timing and luminance of the light-emitting element in the display portion.
The pixel circuit includes a switching element for retrieving video signals and a drive transistor for controlling the current of the light-emitting element as a basic structure. The gate voltage of the drive transistor is controlled by video signals, and controls luminance by flowing drain current into the light-emitting element based on the gate voltage. The drive transistor is realized by a thin film transistor formed by semiconductor thin film. In the pixel circuit, basically, the characteristics of the drive transistors of each of the pixels are assumed to be uniform. If the drain current of the driving transistor is uniformly set by the gate voltage, the luminance of the light-emitting element is made uniform, and by this an accurate gradation display is realized.
However, the threshold voltage of the thin film transistor is difficult to control during the manufacturing process, and normally varies from pixel to pixel. The fluctuation of the threshold voltage of the drive transistor influences the drain current, and as a result causes the luminance of the light-emitting element to vary. In a display device, a variation in the luminance of each pixel is visible as an uneven display. For this reason, in the pixel circuit of the display device, a circuit for compensating for the threshold voltage of the drive transistor provided in each pixel is necessary (for example Japanese Laid-Open Patent Publication No. 2005-031630).
Another problem of the display device in which pixels are configured in the light-emitting element is that luminance deteriorates when the characteristics of the light-emitting element itself vary. The light-emitting element has the problem of varying luminance and a decrease in luminance over time, even if the constant current is supplied. In order to prevent decreased visibility due to varying luminance and deterioration of luminance of the light-emitting layer, a display device provided with a photoelectric conversion element in the pixels is disclosed (see Japanese Laid-Open Patent Publication No. 2005-300897 and Japanese Translation of PCT International Application Publication No. 2003-536115). This display device is provided with a feedback type pixel circuit in which a photoelectric conversion element receives part of the light emitted by the light-emitting element and which controls the drive current of the light-emitting element in response to luminance.
According to the display device disclosed in Japanese Laid-Open Patent Publication No. 2005-031630, in order to configure a threshold voltage compensation circuit, six transistors and one capacitive element are necessary for one pixel. Since it is necessary for this pixel circuit to be provided with many elements in one pixel, the problem of the surface area necessary for forming one pixel becoming greater arises.
According to the display device disclosed in Japanese Laid-Open Patent Publication No. 2005-300897 and Japanese Translation of PCT International Application Publication No. 2003-536115, since the drive current of the light-emitting element is controlled by a transistor, it is necessary to provide a threshold voltage compensation circuit in the pixel for compensating for the threshold voltage of this transistor.
Since it is necessary to provide many elements for one pixel in order to stabilize the luminance of the light-emitting element and achieve improved image quality, it is difficult to achieve high-definition conventional display devices using light-emitting elements.